Fix an annoying bug in tar. When it converted 'tar zcf' to 'tar -z -c -f'
it forgot to null terminate the new argv[] array. If you mixed this
with $TAR_OPTIONS, phkmalloc and a whole bunch of other variables, you
could end up with a segfault. This isn't strictly a phkmalloc victory
since tar walks off the end of an array rather than use uninitialized
malloc memory, but phkmalloc makes it easier to provoke.